diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 3cc61b167ba98..063db05665af1 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -8811,8 +8811,10 @@ CheckPrintfHandler::checkFormatExpr(const analyze_printf::PrintfSpecifier &FS,
       case ArgType::Match:
       case ArgType::MatchPromotion:
       case ArgType::NoMatchPromotionTypeConfusion:
-      case ArgType::NoMatchSignedness:
         llvm_unreachable("expected non-matching");
+      case ArgType::NoMatchSignedness:
+        Diag = diag::warn_format_conversion_argument_type_mismatch_signedness;
+        break;
       case ArgType::NoMatchPedantic:
         Diag = diag::warn_format_conversion_argument_type_mismatch_pedantic;
         break;
diff --git a/clang/test/Sema/format-strings-signedness.cpp b/clang/test/Sema/format-strings-signedness.cpp
new file mode 100644
index 0000000000000..66bf1de1ec09a
--- /dev/null
+++ b/clang/test/Sema/format-strings-signedness.cpp
@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -verify -Wformat -Wformat-signedness %s
+// RUN: %clang_cc1 -triple=x86_64-pc-win32 -fsyntax-only -verify -Wformat -Wformat-signedness %s
+
+// Verify that -Wformat-signedness alone (without -Wformat) trigger the
+// warnings. Note in gcc this will not trigger the signedness warnings as
+// -Wformat is default off in gcc.
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -verify -Wformat-signedness %s
+// RUN: %clang_cc1 -triple=x86_64-pc-win32 -fsyntax-only -verify -Wformat-signedness %s
+
+// Verify that -Wformat-signedness warnings are not reported with only -Wformat
+// (gcc compat).
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -Wformat -verify=okay %s
+
+// Verify that -Wformat-signedness with -Wno-format are not reported (gcc compat).
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -Wformat-signedness -Wno-format -verify=okay %s
+// RUN: %clang_cc1 -triple=x86_64-pc-linux-gnu -fsyntax-only -Wno-format -Wformat-signedness -verify=okay %s
+// okay-no-diagnostics
+
+// Ignore 'GCC requires a function with the 'format' attribute to be variadic'.
+#pragma GCC diagnostic ignored "-Wgcc-compat"
+namespace GH161075 {
+template <typename... Args>
+void format(const char *fmt, Args &&...args)
+    __attribute__((format(printf, 1, 2)));
+
+void do_format() {
+  bool b = false;
+  format("%hhi %hhu %hi %hu %i %u", b, b, b, b, b, b);  // expected-warning {{format specifies type 'unsigned char' but the argument has type 'bool', which differs in signedness}}
+}
+} // namespace GH161075